This invention relates to the field of signal processing. More particularly, the present invention relates to methods and apparatus for restoring a DC level and accurately finding a synchronization pulse in a noisy signal.
The present invention is conveniently described in terms of restoring the DC (direct current) level (commonly referred to as the black level or blank level when discussing video signals) of a signal such as a television signal and detecting a synchronization pulse present in such a signal. However, as will be clear to those of skill in the art from the teachings herein, methods and/or apparatus according to the present invention have applications in other signal processing applications.
Restoring the DC (direct current) level of a capacitively coupled signal such as a television signal is a well-known problem with many proposed solutions.
Viewed from one perspective, DC restoration and finding the HSYNC synchronization pulse in a television signal together form a chicken-egg problem pair. In standard television signals, where the HSYNC start timing is known, DC level restoration (sometimes referred to in the art as xe2x80x9cDC clamping,xe2x80x9d but herein referred to as restoration) can be done easily by timing from a horizontal synchronization pulse (HSYNC), due to the known timing relationship between HSYNC and where the black level in the input is located. However, in order to determine HSYNC start timing, it is generally desirable to know, at least to a close approximation, the DC level.
In some prior art systems, a variety of consecutive approaches have been used to address the twofold problem of determining the DC level and the start of the HSYNC pulse. Some of these approaches are referred to in the art as: passive clamping, HSYNC slicing, clamp gate generation, final clamping, etc. In some situations, passive clamping alone may be used. However, to achieve higher quality clamping several of these approaches may be used together. While these approaches work in some situations, especially in signals with little noise, problems can arise in real-world noisy signals.
A challenge for performing DC restoration in consumer television applications is the high noise to signal ratio these applications may encounter. Attempting to clamp a noisy video signal to the HSYNC tip (or bottom) can cause clamping to a noise value. This can lead to transposition of the noise spectrum into the low frequency domain, thus interfering with the horizontal scanning frequency and making that noise visible. Passive clamping additionally can cause clamping to noise spikes when the amplitude of the noise goes lower than the HSYNC tip.
There is a large body of art directed to receiving composite signals in general and directed specifically to DC restoration of a video composite signal. This art includes analog techniques, digital techniques, and hybrid techniques.
What is needed is a technique that can efficiently and effectively provide level restoration to a signal and detect a synchronization pulse even in the presence of noise.
The present invention involves reducing the susceptibility of a level restoration and pulse detection to noise. According to a further embodiment, a video signal is prefiltered by a low pass filter, and then HSYNC is sliced and may be recovered in a PLL, as is generally known in the art. Timed from HSYNC, a digital level restoration gate signal is generated and is used to activate a digital level restoration.
In a further specific embodiment, the invention provides increased noise immunity of a passive clamp in combination with digital level restoration with a high degree of noise suppression.
In a further aspect of embodiments of the invention, a control signal controls a passive clamping current source. In one embodiment, an output generated from the clamp current source rises gradually when clamping is active and when clamping is inactive, falls off quickly.
The present invention may be understood in the context of receiving a signal. An important application for the present invention, and an independent embodiment, is decoding a video signal, for example in a consumer television. However, using the teachings provided herein, it will be understood by those of skill in the art, that the methods and apparatus of the present invention could be advantageously used in other situations where a signal requires level restoration.
It will be further understood to those of skill in the art from the teachings herein that the present invention can be adapted and embodied in a number of types of digital processing systems, including all digital systems, hybrid systems, etc. The invention can also be embodied into consumer electronic devices such as televisions, video recorders, computers, etc. The invention can further be embodied as a method operating in a logic environment such as a general purpose signal process or simulator.
The invention will be better understood with reference to the following drawings and detailed descriptions. In different figures, similarly labeled items are intended to represent similar functions or signals within the scope of the teachings provided herein. In some of the drawings and detailed descriptions below, the present invention is described in terms of the important independent embodiment of hybrid circuitry that may be used in a video receiver to process video signals. This should not be taken to limit the invention, which, using the teachings provided herein, can be applied to other signal processing situations.